1. Field of the Invention
The present invention relates to therapeutic radiation apparatuses of the type wherein monitoring of the therapeutic radiation exposure field in the subject is undertaken by diagnostic imaging of the subject.
2. Description of the Prior Art
Like surgery and chemotherapy, radiation therapy is an essential means of tumor therapy. Normally high-energy x-ray radiation or gamma radiation is used that penetrates the body, so as to reach a deep-lying tumor and thereby deliver energy to the tissue of the tumor. With the exposure of the tumor to a radiation beam that is directed at the tumor from different directions in the course of the exposure time, it can be achieved that the tumor is always exposed during the duration of an exposure and the tissue in front of and behind the tumor (including the skin) are exposed only intermittently. In this manner a high radiation dose can be applied in the tumor with the therapeutic exposure apparatuses while the other irradiated tissue receives only a fraction (and as slight a fraction as possible) of the radiation dose as the tumor.
Newly emerged diagnostic methods (such as computed tomography), which also enable a more precise exposure planning have already served for more than thirty years in the development toward maximization of the radiation dose in the tumor tissue and minimization in the tumor-free tissue. With the requirement of increased exposure precision, the possibility has additionally developed to monitor the adherence to the exposure field set according to the plan by means of imaging during the exposure process.
After it has passed through the patient body, the therapeutic radiation beam proceeds to the image-acquiring system. In the simplest case this is a radiographic film system that is formed of a film and a radiation-converting luminophore foil that for its part exposes the film.
Systems of this type have been relied on for more than twenty years since the beginning of the imaging exposure field monitoring, known as portal imaging generation of or a verification image for the exposure field. The radiographic images generated with these systems and depicting the exposure field naturally have unsatisfactory properties with regard to contrast and sharpness in comparison to the images generated with a diagnostic x-ray system. This is due to the high energy of the therapeutic radiation (that, for example, lies in the range of a few MeV) and interaction of the focal spot of the radiation source (which focal spot is a few millimeters in size) with the radiation geometry provided by the exposure apparatus. The disadvantageous absorption or image conversion properties of the radiographic image system for this high-energy radiation to which it is exposed represent additional problems.
In spite of the insufficient image quality, such images are usable for the monitoring of the exposure field and the adjustment thereof with regard to the body of the patient, so that such systems have been developed and improved, for example by the use of luminophore-coated metal foils that offer a higher quanta yield and thereby improve the image quality. Use was made of the fact that, through the absorption of quanta in metal, electrons released from this metal contribute to the exposure of the film by light excitation in the luminophore.
FIG. 1 shows the basic design of a known exposure apparatus of the type described above. The patient 1 is borne on a table plate 3 that can be displaced at the foot 2 of a patient bed, and that is height-adjustable. The exposure apparatus includes the base 5 (firmly connected with the floor 4) that carries an extension arm 7 that can be rotated around the rotation axis 6, on which extension arm 7 the radiator 8 is mounted. The radiation beam 10 is shaped (in terms of its cross-section) by a collimator 9 and exits from the collimator 9, which is associated with the radiator 8. The patient 1 is positioned with the table plate 3 such that the radiation beam 10 penetrates a tumor located in the shoulder region in the depiction; the tumor lies in the rotation axis 6 of the exposure apparatus. The radiation beam 10 penetrating the patient exits the opposite side of the patient 1 as a radiation beam 10′ after surrendering energy in the body. It then strikes the radiation detector 11 (having two edges 11′ appearing in the side view) at the top side, at which it is absorbed. The environment of the exposure apparatus is thus protected from the radiation beam 10′ exiting from the patient, which otherwise would freely radiate in space depending on the position of the extension arm 7.
However, the radiation detector 11 shown in FIG. 1 can serve not only for radiation capture but (according to FIG. 2) can also serve as a support (carrier) for an imaging system 12 that is shown with its upper edges 12′ in an angled position of the extension arm 7. In the simplest case this system 12 is a radiographic film system is formed of a radiographic film in contact with a radiation-converting foil that exposes the film in addition to the radiation directly absorbed thereby. The radiation beam 10′ striking the imaging system 12 is the radiation beam 10 that was modulated upon passage through the patient 1 corresponding to the radiation attenuation properties of the traversed body segment, and therefore records on the film of the imaging system 12 a radiogram of the respective body section acquired by the radiation beam 10.
The x-ray or gamma radiation emitted by the radiator 8 usually generated by a linear accelerator 13 shown in FIG. 3. In this accelerator 13, an electron beam 15 exits from an electron gun 14, the electrons, the electron beam 15 are accelerated to high energies (for example 6 MeV) in the axis 16 of the waveguide structure 17 fed with radio-frequency energy by its axial electrical field.
These electrons of high energy exit from the waveguide structure 17 through the vacuum window 18 at the end of said waveguide structure 17 and strike the target 19 (which is a thin disc made from suitable heavy metal). In this target 19 the electron beam 15 generates high-energy x-rays or gamma rays that exit from the focal spot 20 on the side of the target 19 facing away from the electron beam 15 and form a radiation beam 21. The recumbent board 21 is brought into the position needed for the exposure by the primary collimator 22 and the secondary collimator 23, the latter corresponding to the collimator 9 shown in FIG. 1 and FIG. 2. The radiation beam 21 thus becomes the radiation beam 10 that enters into the patient 1 according to FIG. 1 and FIG. 2. FIG. 3 shows a compensation filter 24 between the primary collimator 22 and the secondary collimator 23. This compensation filter 24, due to its shape in the core of the radiation beam 21, reduces the excessive radiation power there (due to the given type of radiation generation) and therefore normalizes this power across the cross-section of the radiation beam 21. The secondary collimator 23 (shown in FIGS. 1 and 2 as collimator 9) has the task of removing the diffuse edge zones of the cross-section of the radiation beam 21 that are left by the primary collimator 22 due to the beam geometry before the radiation beam 21 leaves the radiator 8 as radiation beam 10.
FIG. 3 shows only the basic components of the radiator 8. Dose measurement chambers (such as, for example, structural elements for deflection of the accelerated electron beam 15) that enable advantageous designs of the radiator 8 are not shown.
Since the development of the system described above, therapeutic radiation exposure apparatuses have experienced further improvements, for example with regard to collimators for more sharply defining the edges of the exposure field and also with regard to the possibility to continuously alter the radiation power in the exposure field dependent on the radiation direction by adaptation to the changing projections of the tumor and the tissue to be spared, if at all possible, by the radiation.
The means for imaging diagnostics have likewise increasingly had a higher precision for diagnostics and therefore also localization of sources to be irradiated in accord with a steadily improving therapy planning.
The requirements for manageability, precision and reliability of the exposure field monitoring or just the portal imaging, therefore have increased, with the image quality being accorded a significant role. Manageability has played a role because, to assess the film in the imaging system according to FIG. 2, this had to be developed first and in a time-consuming manner. A first step for improvement of the exposure field monitoring for the imaging system 12 according to FIG. 2 was to replace the radiographic film system with a radioscopy system that passes the light density (luminance) generated thereby due to the radiation to a system that is formed by an optical image intensifier and a downstream television system. The relaying of the light density image to the image intensifier via a mirror allowed radiation-sensitive parts of the electronics of the image intensifier-television system to be arranged outside of the therapeutic radiation beam.
However, the decisive improvement of the image quality of the portal imaging occurred by the use of regular diagnostic x-ray radioscopy devices to irradiate the body sections involved in the therapeutic radiation. For instance, in the first half of the 1990's, exposure devices were known in which the radiator of the diagnostic x-ray system was firmly connected with the radiator 8 (shown in FIGS. 1 and 2) of the exposure apparatus, such that the central rays of the therapeutic radiation beam 10 as well as the diagnostic rays strike in what is known as the isocenter (defined by the rotation axis 6 of the exposure apparatus) and move in small rotary movements, and enclose an optimally small angle, such as angles of 37° and 45°, for example. Naturally identical projections for the therapeutic and diagnostic radiation beams can be realized in principle here because of the identical beam geometries. Such identical projections, occur only in succession (thus not be simultaneous) with regard to the radiated body sections. For an exposure field monitoring either the depth diaphragm of the diagnostic system must emulate the collimator for the therapeutic radiation, or an emulation of the therapeutic exposure field must be superimposed on the image acquired with the diagnostic radiation beam under consideration of the respective different positions of the two central rays. An advantage of such an arrangement is that it offers the possibility for the diagnostic system to be equipped with a conventional x-ray intensifier that, like the connected image electronics, remains outside of the therapeutic radiation beam.
A radiation therapy apparatus is described as a concept in “Imaging Systems for Medical Diagnostics”, edited by A. Oppelt; Editor: (Siemens Aktiengesellschaft; Publisher: Publicis Corporate Publishing, Erlangen 2005, chapter 17.1 “Imaging for radiation therapy”) that represents an extension of FIG. 1 herein. This known radioscopy system is shown in FIG. 4 and has an x-ray radiator 25 for diagnostic radiation with an associated depth diaphragm or collimator 26 and an image-converting detector 28. The x-ray radiator 25 is mounted at the exposure apparatus by a crossbar 27 such that central ray of the emitted diagnostic radiation beam 29 is congruent with the central ray of the therapeutic radiation beam 10 according to FIGS. 1 and 2, but is directed in the opposite direction and therefore along the axis 16. The radiation beam 29′ is the continuation of the radiation beam 29 after passage through the patient 1.
The detector 28 of the type known as a flat panel detector in which a luminophore layer converts the x-ray radiation into a light (luminescent) image that is in turn transduced by an array composed of amorphous silicon into electrical signals. An advantage of such a flat panel detector is that it can be irradiated without damage by the therapeutic radiation when only the associated electronics remain outside of this radiation.
The radioscopy system of the radiation therapy apparatus in FIG. 4 can therefore simultaneously irradiate the same body section as the therapy beam, but as shown in FIG. 4 this occurs in the opposite direction. Although the acquired body sections are therefore nearly identical, they can be non-identical due to the central projections in opposite directions. As used herein, simultaneously (or, more precisely, quasi-simultaneously) means that the radioscopy system is pulsed so that its radiation pulses are emitted in the time gaps of the pulsed therapeutic radiation.
The result is that, given the use of the described diagnostic x-ray systems for exposure field monitoring, the image quality accommodates the precision requirements for radiation therapy, but with the sacrifice of dispensing with the possibility to simultaneously acquire identical projections for the therapeutic and the diagnostic radiation beam in a given position of the exposure apparatus, which was provided in the first place in portal imaging from its inception. A particular advantage was that the image produced for the exposure field monitoring likewise identically showed the exposure field from the outset because the image according to FIG. 2 was produced with the radiation beam 10′ exiting from the patient 1.
If portal imaging in its original form is therefore excluded from discussion due to insufficient image quality, the question remains as to which system should preferably be used: a system that offers a practically simultaneous verification image for the exposure field, but with a different distortion in comparison to an image of the original portal imaging (because, according to FIG. 4, the therapeutic radiation and diagnostic radiation pass through the body section irradiated by them in opposite directions), or a system with an image with a projection identical to the exposure field, but that shows the current exposure field with a time offset. In both cases the diagnostic radiation beam for the imaging has its own collimator or gating device that is thus not identical with the collimator device for the therapeutic radiation.